Study on Microplastic Distribution and Source-Sink at the Sediment-Water Interface in the Three Gorges Reservoir

Rivers are generally recognized as crucial pathways for transporting microplastics from terrestrial environments to the oceans. However, water-retaining structures such as dams disrupt river continuity, creating conditions for the temporary storage and retention of microplastics at the sediment-water interface (SWI). To clarify the occurrence, migration, and exchange patterns of microplastics at the SWI under the influence of Three Gorges Reservoir (TGR) operation, we collected samples at three different interface depths from 30 sites in the main stream of the TGR and conducted a comprehensive assessment of microplastics therein. The results showed that a significant vertical concentration gradient attenuation of microplastics existed from the fluctuating backwater area at the reservoir tail to the perennial backwater area at the reservoir head. The reservoir tail, coupled with high-intensity anthropogenic input and the retention effect of the backwater area, became a "hotspot" for microplastics. Meanwhile, microplastics in the bottom water and surface sediments exhibited strong dynamic transformation characteristics: the microplastic concentration in surface sediments reached the highest during the high water level maintenance period and decreased significantly during the low water level maintenance period. The size, morphology, and polymer types of microplastics varied greatly with sampling periods and depths. Hydrological conditions significantly altered the concentration and characteristic composition of microplastics at the SWI, and the "source-sink" role of sediments shifted with the operation rhythm. Furthermore, the intrinsic properties of microplastics (morphology, size, polymer type) and environmental medium conditions (flow velocity, suspended sediment concentration) synergistically affected their distribution at different depths. These findings highlight the importance of reservoir rhythm in shaping microplastic distribution and underscore the dual role of sediments in the microplastic migration cycle. These microplastics may pose varying degrees of threats to benthic organisms, aquatic food webs, and groundwater quality during different reservoir operation periods.